Jet airplanes are built to operate at high speeds during flight. This results in relatively high speeds during landing. These speeds pose a challenge when trying to bring an airplane to a full stop after a landing, or during a refused takeoff. The plane's landing gear brakes and wing aerodynamic devices are the primary items used to slow the airplane. Assisting these items are thrust reversers that slow the airplane further by redirecting either engine exhaust gases or engine fan air in a nonrearward direction. Typically, thrust reverser operation is initiated by the pilot shortly after the airplane has touched down on the runway.
Thrust reversers come in a variety of designs depending on the engine manufacturer, the engine configuration, and the propulsion technology being utilized. Thrust reversers for turbofan engines are typically reversed in three ways. Cascade-type reversers, which redirect fan flow air through cascade vanes positioned on the periphery of the engine, are located at the engine's midsection. Cascade-type reversers are normally used on high bypass ratio engines. Target-type reversers, sometimes called clamshell reversers, utilize two doors to block the entire jet efflux. These doors are in the aft portion of the engine and form the rear part of the nacelle. Target reversers are typically used with low-bypass ratio engines. Pivot door reversers are similar to cascade-type reversers except that no cascade vanes are provided. Instead, four doors on the nacelle blossom outward to redirect flow.
As will be better understood from the following description, the present invention is a locking mechanism, specifically a thrust reverser synchronization shaft lock, that is ideally suited for use with hydraulically controlled cascade-type thrust reversers. While ideally suited for cascade-type reversers, it is to be understood that the present invention may be adapted for use with other thrust reverser systems that are hydraulically controlled and have a synchronization mechanism as a component of their thrust reverser systems.
A cascade-type thrust reverser works as follows. Thrust reverser sleeves (sometimes called translating cowls) are positioned circumferentially on the outside of the engine and cover cascade vanes (i.e., nonrearwardly facing air vents). The cascade vanes are positioned between the thrust reverser sleeves and the fan airflow path. A series of blocker doors mechanically linked to the thrust reverser sleeves are located in the fan airflow path. In their stowed position, the blocker doors are parallel to the fan airflow. In their deployed position, the blocker doors are transverse to the fan airflow path. When the thrust reversers are activated, the thrust reverser sleeves slide aft, causing the cascade vanes to be exposed and the blocker doors to move into their deployed position. This further causes the fan air to be redirected out the cascade vanes. The redirection of the fan air in a forward direction works to slow the airplane.
The thrust reverser sleeves are operated by one or more actuators per engine. The actuators are located on the fan case of the engine and interconnect with each other via a synchronization mechanism, such as a flexible shaft. The synchronization mechanism ensures that the actuators move at the same rate.
Thrust reversers are controlled by a hydraulic thrust reverser control system. Signals transmitted from the flight deck to the control system determine the desired state of the thrust reversers. The actuation system includes components that receive the signals and use them to regulate the pressure in various hydraulic fluid lines. Hydraulic pressure controls the position of the thrust reverser sleeves, guiding them between their deployed and stowed states. The hydraulic fluid lines may be hydraulically independent from the rest of the airplane, or may use hydraulic fluid that is a part of a larger airplane hydraulic system. The configuration of the latter, therefore, relying on supply and return lines from the overall airplane hydraulic system, as opposed to the former, which must include components to create internal supply and return lines. In either case, the supply line is designed to be at a higher pressure than the return line.
Typically, thrust reversers (hydraulic or otherwise) employ locking mechanisms to ensure that the thrust reversers are only activated at the proper time. Most locking mechanisms accomplish this by not allowing the actuators to deploy the thrust reverser sleeves unless a deploy command is initiated. In other words, these locking mechanisms only unlock the thrust reversers when a deploy command occurs.
Other locks simply operate to stop the internal and external forces acting on the thrust reversers from pushing the sleeves into their deployed state. One example is the lock described in U.S. Pat. No. 4,586,329 to Carlin. This patent describes a mechanical antirotation device to prevent uncommanded deployment of the thrust reversers due to gas loads within the engine and air loads external to the engine. The antirotation device is a mechanical lock attached to the synchronization shaft that interconnects mechanical jackscrews.
A feature of current locking mechanisms is that the actual locking elements tend to comprise a pin holding a toothed gear in a stationary position. The Carlin patent, discussed above, is a typical example of such a device where the engagement forces of the locking components are concentrated at a single pin pressed against a single abutment.